1. Field of the Invention
This invention relates to an original reading apparatus such as a facsimile or the like.
2. Description of the Prior Art
In recent years, compact original reading apparatus comprising solid sensors and a minute lens array have been developed in the field of facsimile. Examples of such reading apparatus are shown in FIGS. 1 and 2 of the accompanying drawings. FIG. 1 is a schematic view of a reading apparatus using solid sensors and a one-layer lens array. In FIG. 1, reference numeral 1 designates an original placed on an original supporting table, not shown, reference numeral 2 denotes minute lenses, and reference numeral 3 designates solid sensors such as CCD or photodiode arrays. The original 1 is conveyed in a direction perpendicular to the plane of the drawing sheet and partial inverted images of the original are formed on the sensors 3 by the individual minute lenses 2. The arrangement direction of the solid sensors 3 corresponds to the line of one main scanning direction of the original 1, and the outputs from the solid sensors are rearranged in the order of the image element rows of the original, whereby reading of one line of the original is effected.
FIG. 2 is a schematic view of another example using a solid sensor and a minute lens array. In FIG. 2, reference numerals 4 and 6 designate imaging lenses and reference numeral 5 denotes field lenses. Reference numeral 7 designates a one-dimensional long solid line sensor. The original 1 is conveyed in a direction perpendicular to the plane of the drawing sheet and erect images of various portions of the original formed by the lenses 4, 5 and 6 are connected, whereby a complete image of one line is formed on the sensor 7. In this example, unlike the example shown in FIG. 1, the image of the original is an erect image and therefore, if the bit outputs of the sensor 7 are successively taken out, they provide signals of one line of the original 1 in that order.
The plural solid sensors 3 in the example of FIG. 1 or the solid sensor 7 in the example of FIG. 2 is arranged over a length equal to the width of the original 1 and the length of the light path of each individual minute lens may be short and therefore, the reading apparatus is very compact.
However, such a minute lens array, as compared with a single lens, is difficult to prepare. This is because, in order to accurately maintain the relative positions of the minute lenses, for example a lens array comprising a plurality of minute lenses arranged on the same base (which is not always formed of a single material) must be formed.
As the material of the lenses used in such an optical system, plastic is desirable because of its low cost and ease of manufacture. However, plastic has its properties readily variable for variations in environmental conditions and particularly has a great absorbing characteristic which leads to a fatal drawback that any variation in humidity would cause entry of moisture from the peripheral portion of the lenses which in turn would result in variation in refractive index. Thus, the individual lenses themselves have refractive index distributions and further, in an ommateal system, the lenses differ in refractive index from one another depending on the positions at which they are arranged, whereby the imaging performances of the lenses fluctuate to preclude stable reading of an original. Such disadvantage of plastic has made it difficult to put into practical use a reading apparatus using a plastic lens array.